1. Field of the Invention
The present invention generally relates to waveguide bandpass filters, and more particularly, to waveguide structures which realize the general coupled cavity transfer function in the high Q circular TE.sub.011 mode.
2. Description of the Prior Art
High-quality microwave communications system applications require narrow-bandpass filters possessing good frequency selectivity, linear phase, and small in-band insertion loss. Although direct coupled resonator filters are relatively simple structures, their insertion loss functions are restricted to all-pole functions, e.g. Butterworth or Tchebychev functions. The applicants have shown that optimum waveguide bandpass filters whose insertion loss functions have ripples in the passband and real finite zeros of transmission in the stopband, can be constructed by using dual-mode multiple coupled cavities. See A. E. Atia and A. E. Williams, "Narrow-Bandpass Waveguide Filters," IEEE Transactions on Microwave Theory and Techniques, Vol.MTT-20, No. 4, April 1972, pp. 258-265. These filters still require separate group delay equalizers, however.
Since it is known that cascading a non-minimum phase network with an all-pass network results in a network of a higher degree than is actually necessary for a particular application, direct realization of a general non-minimum phase transfer function would offer considerable advantages. Unfortunately, the existing analytical solution to the approximation problem of optimizing both the amplitude and phase responses of a filter transfer function over the same finite band of frequencies does not yield the most optimum characteristics. The existing analytical solution to the approximation problem is described by J. D. Rhodes, "A Low-Pass Prototype Network for Microwave Linear Phase Filters," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-18, No. 6, June 1970, pp. 309-313. See also U.S. Pat. No. 3,597,709 to J. D. Rhodes. While Rhodes' waveguide realization of the linear phase filter produces excellent group delay response, its monotonic out-of-band amplitude characteristics are far from optimum. Moreover, Rhodes' theory does not contemplate the realization of an elliptic function bandpass filter.
U.S. Pat. No. 3,697,898 to B. L. Blachier and A. R. Champeau describes a plural cavity bandpass waveguide filter which provides an elliptic function response. The Blachier and Champeau filter employs a plurality of waveguide cavities each of which resonate in two independent orthogonal modes. Such cavities may be realized by using either circular or square waveguides. Coupling within the cavities is provided by structural discontinuities such as a screw, and coupling between cavities is provided by a polarization discriminating iris. The coupling is such as to produce a phase inversion and hence subtraction between selected identical modes in coupled cavities thereby providing the steep response skirts for the passband of the filter which are characteristic of the elliptic function.
A particular advantage of the Blachier and Champeau filter is that it provides superior filter characteristics in a limited volume; both factors which are very important in satellite and space applications. Dual mode cavities, however, require more precise machining than single mode cavities, and when used in the Blachier and Champeau filter, also require intra cavity mode coupling.
Filters constructed from rectangular, square or circular cavities are typically designed to oscillate in the fundamental TE.sub.101 or TE.sub.111 modes, respectively. For silver-plated waveguide cavities at 12 GHz, unloaded Q's of 5500 are usually obtained. However, for specific applications such as satellite transponders where a channelizing set of narrow band filters are required, such a Q may not be adequate, especially if the bandwidths are less than 1%.
The obvious way to improve the realizable filter unloaded Q is to employ a higher order cavity mode, although practical problems related to the control of lower order modes are introduced. Nevertheless, one mode which has been successfully employed is the circular TE.sub.011 mode. Direct coupled cavity bandpass filters have been constructed having practical Q's of at least three times those of the fundamental mode. See, for example, Matthaei, Young and Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures, McGraw-Hill Book Company (1964), pp. 924-934. These filters will realize Butterworth, Tchebychev or augmented linear phase filter functions, i.e., those functions which can be generated with all positive (or all negative) intercavity coupling. Filter transfer functions having real zeros of transmission or filters having negative coupling are not possible with such a structure and have not been previously described in literature.